The invention relates to inhibited emulsion explosives, and more particularly to zinc oxide inhibited emulsion explosives.
U.S. Pat. No. 5,159,153 (Cranney et al.), incorporated herein by reference, acknowledged the well known problem that certain sulfide/pyrite ores are reactive with ammonium nitrate, and that when a blasting agent containing ammonium nitrate is used in a borehole, heat generated in the borehole by such reactivity can cause premature detonations. The Cranney et al. patent refers to U.S. Bureau of Mines Reports of Investigation Nos. 7187 (hereinafter, "Report 7187") and 8373 (hereinafter, "Report 8373") and U.S. Pat. Nos. 3,447,982 and 3,708,356 as disclosing use of urea to inhibit any reaction of AN (ammonium nitrate) or ANFO (ammonium nitrate and fuel oil) with reactive ores.
Cranney et al. specifically teach that the oxidizer salt ammonium nitrate, urea and other aqueous soluble constituents are preferably first dissolved in the water or an aqueous solution of water and miscible liquid fuel.
Report 7187 (1968) disclosed that the addition of 5 percent acid to ANFO reduced the reaction temperature from 340.degree. to 228.degree. F. The temperature of incipient reaction of mixtures of AN and pyritic ore was similar to that of mixtures of ANFO and ore, both dry and with water or acid, except that the reactions of the oil-free mixtures were more exothermic. Report 7187 also disclosed that mixtures of "AN-ore" and "AN-FO ore" with 0.5 and 1.0% urea, both dry and with 5 percent water elevated the reaction temperature from 185.degree. F. to 350.degree. F. Referring to a study by Rozman "On the Rational Behavior of Inhibitors on the Thermal Decomposition of Ammonium Nitrate", J. Appl. Chem., U.S.S.R., June 1960, pp. 1258-1263 of thermal decomposition of AN by Rozman, Report 7187 disclosed that the addition of urea, zinc oxide, or other basic salts to the AN-sulfur-potassium chloride mixture stabilized the exothermic reaction to about 550.degree. F., probably due to inhibition of acid-catalyzed exothermic reactions. Thus, Report 7187 disclosed that urea effectively inhibits reactivity of ammonium nitrate prills (AN), ammonium nitrate in ANFO, and ammonium nitrate in aqueous solution.
Report 8373 (1979) discloses that 5 percent by weight of urea was found sufficient to prevent a reaction among ammonium nitrate, fuel oil, and ferrous sulfate, which is a product of weathering of certain sulfides and pyrites. Smaller amounts of the urea and potassium oxalate slowed down the reaction and delayed its onset to higher temperatures, but did not prevent it. Report 8373 also discloses the main chemical reaction that needs to be inhibited. The Bureau of Mines Reports 7187 and 8373, and the two Rozman articles cited therein disclose that the reaction occurs in the presence of pyrite ore including ferrous sulfate and ammonium nitrate in any form, irrespective of whether the ammonium nitrate is solid, is wetted by 6% fuel oil (as in ANFO), or is in aqueous solution (as in emulsion explosives).
According to Report 8373, five weight percent urea completely prevented an exothermic reaction among the ingredients when the test mixture was heated up to 180.degree. C., and indicated that (1) urea inhibits reaction between AN-FO and ferrous sulfate by combining with one or more of the ingredients in the original reaction mixture, (2) only when enough urea is present is reaction prevented, (3) urea can undergo many reactions, and replaces water of crystallization in many salts, including sulfates, thereby forming new compounds with different stabilities than the original salts, and (4) urea also forms adducts with paraffin hydrocarbons (such as fuel oil).
Thus, it was well known that urea is effective in inhibiting the reaction of the nitrate radical NO.sub.3.sup.- with the ferrous ion Fe.sup.++ to generate heat, which then can lead to autocatalytic exothermic decomposition of ammonium nitrate and cause premature detonation. It also was known that regardless of whether the ammonium nitrate is dry, wetted with fuel oil, or in aqueous solution, the nitrate radical NO.sub.3.sup.- therein is available to react with certain sulfides and the ferrous ion.
A third Bureau of Mines Report No. 8727 (hereinafter, "Report 8727") published in 1982 discloses that (1) urea also combines with acids, but (2) in the opinion of the authors of Report 8727 it is urea's ability to undergo additional reactions that makes it the best inhibitor of the ones tested. If urea is added as a dry prilled ingredient at sufficient levels to be effective as an inhibitor in emulsion explosives, i.e., approximately 3 to 5 weight percent, then the hygroscopic nature of urea causes handling problems. Storage space and weight limitations become problematic, particularly with mobile manufacturing equipment used at borehole sites. Another drawback to adding prilled urea as a dry ingredient in emulsion explosives is the fact that at economical levels, solid urea is not well dispersed therein and a relatively large amount of the urea is not in contact with the emulsion or the pyrite ore. Consequently, an undesirably large portion of the emulsion explosive is not stabilized and there may continue to be a significant risk of a premature detonation. Even though urea prills could be ground up to provide small particle sizes which would be more uniformly dispersed in the emulsion explosive, the extremely hygroscopic nature of the urea particles would make storage thereof very problematic. Grinding urea prills at the time of manufacture is not practical on a mobile manufacturing unit at the borehole site.
It would be desirable to provide an economical inhibited emulsion explosive which avoids the above drawbacks of urea-inhibited emulsion explosives, namely the increase in pH, the storage and handling problems, and the poor dispersal of urea prills in the oxidizer phase.
To summarize the teachings of the known prior art, the earliest Bureau of Mines report concluded that the Butte, Mont. accident on May 16, 1967 could have been prevented by introducing powdered zinc oxide as a coating when mixing prilled ammonium nitrate and fuel oil prior to ejector placement of the coated ANFO into three inch diameter blast holes at the accident site. However, the later Bureau of Mines studies focused on larger diameter vertical boreholes and bulk loaded ANFO. (Note that the Bureau of Mines reports span a period of fourteen years.) Such later studies concluded that urea was a better inhibitor of reaction between ANFO and pyritic ores than zinc oxide because urea was significantly more effective inhibiting the reaction between ANFO and ferrous sulfate, which is usually present in pyrite ore because ferrous sulfate is a weathering product of pyrite ore (iron sulfide). The Rozman articles discussed use of both zinc oxide and urea to inhibit reactivity of ammonium nitrate in aqueous solutions and concluded that urea was preferable because it is soluble in water and therefore completely dispersed in the solution. It is known that zinc oxide is very insoluble in aqueous solution, and that any attempts to use zinc oxide as an inhibitor in an aqueous solution of ammonium nitrate would require complicated and expensive efforts to keep the fine zinc oxide powder in suspension prior to passing the mixture through a high shear emulsifier.